The retinal nerve fiber layer is the innermost layer of the retina, and consists of the nerve fibers (ganglion cell axons) which transmit the visual signal from the photoreceptors to the brain. With the onset of glaucoma and other optic neuropathies, there is increasing damage to the nerve fibers, causing impaired vision or blindness. Glaucoma and other diseases must be diagnosed early on to slow or stop this process, and for an accurate diagnosis it is important to ascertain the presence and extent of any such damage.
Because the nerve fiber layer is very thin (about 30 .mu.m to 150 .mu.m) and the optical depth resolution of the human eye is only about 200 .mu.m to 300 .mu.m, measurement methods based on optical imaging are not sufficient to accurately measure the thickness of the nerve fiber layer. In addition, the retinal nerve fiber layer is transparent, which makes it even more difficult to assess it by imaging means.
Conventionally, assessment of damage in the nerve fiber layer is made with red-free fundus photography. Visible light of shorter wavelength ("red-free") is employed to achieve increased scattering of light within the nerve fiber layer, improving the visibility of the otherwise transparent layer. However, no quantitative measurements of the nerve fiber properties can be obtained with this method.
Other, more indirect methods have been developed to estimate nerve fiber layer thickness. Zeimer (U.S. Pat. No. 4,883,061) described a geometric method that uses the projection of a line onto the fundus. The reflections of the line from the anterior and posterior surfaces of the retina are used to measure the thickness of the total retina which is about 500 .mu.m to 700 .mu.m thick. The resolution of this method is, however, not sufficient to measure the thickness of the nerve fiber layer which is only one thin layer of many layers of the retina, making up possibly one-tenth of the total retinal thickness. Even with as little margin for error as .+-.5%, the measurement error could be as great as the measurement itself.
Another indirect approach to assessing the nerve fiber layer condition is to measure the topography of the internal limiting membrane which forms the anterior surface of the nerve fiber layer. The result of this type of measurement, however, is the measurement of only one surface of the nerve fiber layer. Absolute thickness measurement of the nerve fiber layer is not possible. Topography instruments (i.e. U.S. Pat. No. 4,900,144) employ means of detecting intensity of light reflected from the fundus surface (internal limiting membrane). The systems determine the focus position of maximum light reflection and assume this focus position to be the position of the internal limiting membrane. In reality, however, the light detected by these systems is composed of light deriving from many different retinal layers, and the position of the maximum light reflection is usually displaced rearwardly of the membrane an indeterminate amount, producing false readings.
Geometric techniques alone will not produce clinically meaningful measurements of nerve fiber layer thickness or topography as proven by the results obtained from these techniques. The characteristics of the eye must be probed beyond its geometry, which the instant inventors have done. The result is a measuring apparatus which takes advantage of differences in polarization properties of various layers of the eye to augment or replace geometry-based techniques for relative spatial measurements in vivo, and on the eye.
The present invention further recognizes that the on site collection of data from a patient need not be conducted by a doctor and indeed need not be analyzed on site at all. Instead, as recognized herein it might be desired for convenience and cost purposes that a technician collect the data from a patient, and that the data be sent off-site for analysis.